Monday, March 26, 2018

The MDT Tool

In the MDT tool, unwanted fluid is expelled from the tool using the pumpout module. During sampling, the engineer can monitor the resistivity and temperature of fluid in the flowline while pumping it directly into the borehole or into a dump chamber. When fluid quality is judged to be representative of the reservoir, the pump is stopped and pure formation fluid can be diverted to the sample chamber or, if a sample is not required -often the case when formation water or gas is indicated -another zone can be tested.

To prevent gas from coming out of solution during sampling, pressure is maintained above bubblepoint using throttle valves in the sample chambe controlled by surface software. Maintaining pressure above bubblepoint reduces drawdown, which helps prevent crumbling in soft formations. Excessive drawdown can result in seal failure and hence mud contamination of a sample. Drawdown can also be limited by using a water cushion and choke with the multisample module.

Formations in which seal failures are likely- highly laminated or otherwise heterogeneous formations -or formations that have low permeability can be tested and sampled using the dual-packer module. Instead of a probe and packer to provide a seal, two inflatable packers are used to isolate an interval of about 3 ft of ormation forming a mini drillstem test. The pumpout module is used to inflate the packers and also to expel mud from between the packers before sampling.